The interfacial reactions between Ta-Ni thin films and (100) GaAs substrate have been analyzed by Auger electron spectroscopy, x-ray diffraction, and transmission electron microscopy. A pure Ni film reacts with GaAs at 250°C forming a ternary Ni2 GaAs epitaxial compound, which at 600°C dissociates to NiGa and NiAs. Ta interacts with GaAs only at 650°C. In the case of a Ta/Ni film on GaAs at temperatures up to 450°C only the Ni layer reacts with the substrate forming Ni2GaAs. At higher temperatures the dissociation of Ni2GaAs combined with As outdiffusion results in the structure TaAs/NiGa/GaAs. For the complementary Ni/Ta bilayer, the Ta/GaAs interface is intact up to 500°C; at this temperature only Ni-Ta intermixing takes place. At 600°C, Ni penetrates into the interface with GaAs and forms NiGa. The reaction of the alloy film Ni60Ta 40 with GaAs starts at 450°C by Ni leaching out of the alloy and forming Ni2GaAs at the interface with GaAs. At higher temperatures the reaction results in a structure similar to that obtained for the bilayers with NiGa adjacent to the GaAs substrate. The potential applications of the studied metallization schemes for contacting to GaAs are also discussed.